Hey mate
I had the a simular problem with my 1999 VTII Commodore, I took it to a few mechanics and Holden and it always came back with no fault found. Untill one day I left it with a mechanic overnight and it happened to him, he took to one of his auto electrician friends and it ended up being a cheap $5 relay, the car now it very close back to normal, but I also had the crack angle senor and others replaced. I love my VT and glad to have it back
I hopr this helps
Shaun VTII

love it.